In transport costing, the choice of composite units varies based on the service type. For freight transport, costs are typically quantified using tonne-kilometres or quintal-kilometres, indicating the expense of transporting a specific weight over a given distance. While 'cost per km' is broadly applicable, it's often used internally for vehicle kilometres. However, 'cost per passenger-km' is exclusively for passenger transport services, such as buses or trains, not for carrying goods. Consequently, 'cost per passenger-km' is an inappropriate service unit for a goods transporter. Therefore, the appropriate unit is 'Cost per passenger-km'.